Introduction: An important aspect of thought is the ability to solve novel problems. Very little is understood about the brain basis of fluid reasoning. fMRI was used to identify which brain regions are important for fluid reasoning. We further examined whether there were separable brain region involved in abstract or analytic reasoning versus figural or visuospatial reasoning. We therefore examined brain activation while subjects solved problems from the Raven's Progressive Matrices Test. Methods: Activation was examined in seven, normal right-handed subjects (3 men, 4 woman, aged 23-30 years) with fMRI using blood-oxygen level-dependent contrast measured by a gradient echo spiral sequence. Each subject performed three scans that included (1) analytic reasoning, (2) figural reasoning, and (3) a baseline condition that controlled for perceptual and motor aspects of test performance but did not involve any reasoning. Results: Figural (visuospatial) reasoning activated right frontal and bilateral parietal areas. In addition to those areas, analytic reasoning activated bilateral frontal, and left parietal, occipital, and temporal areas. Further analyses showed that figural and analytic reasoning were disproportionately mediated by right and left hemisphere, respectively, but that both hemispheres contributed to both kinds of reasoning. Also, frontal areas contributed disproportionately relative to posterior cortical areas for analytic reasoning. Thus, reasoning activated an extensive, but specific, network of cortical regions. Therefore, it appears that reasoning on a novel problem activates all kinds of working memory capacities that may maintain a wide variety of relevant information in mind as subjects reason about a difficult problem. Conclusions: These findings constitute the most specific information about the brain basis of fluid reasoning abilities that people use to solve novel problems. Impairments in reasoning are prominent in several disease, including Alzheimer's disease and schizophrenia. Fluid reasoning also declines modestly but steadily in normal aging. Further studies will aim to better understand how age-related or disease-related changes in reasoning relate to changes in brain functions.